Heat the oven to 500°F with a rack in the middle position. In a large pot over high, bring 2 quarts water to boil.

Add the potatoes, garlic, rosemary and ½ cup salt, then cook, uncovered and stirring occasionally, until a skewer inserted into the largest potato meets no resistance, 18 to 22 minutes.

Using a slotted spoon transfer the potatoes to a wire rack set in a rimmed baking sheet (leaving the rosemary behind); place the garlic in a small bowl; remove and discard the rosemary. Let the potatoes cool for about 10 minutes.

Meanwhile, using a fork, mash the garlic to a paste, then stir in the lemon juice and 2 tablespoons of oil, followed by the chilies; set aside.

After the potatoes have cooled slightly carefully remove the rack from the baking sheet. Wipe away any moisture on the baking sheet and place the potatoes in an even layer directly on the sheet.

Using the bottom of a dry measuring cup or ramekin, press down on each potato so it is slightly flattened and splits open but remains intact. Brush the tops of the potatoes with the remaining 4 tablespoons oil.

Roast the potatoes without moving them until browned and crisp, 35 to 40 minutes. Using a wide metal spatula, transfer to a serving platter, then sprinkle with the parsley and drizzle with the vinaigrette.